The present invention relates to a control device of a compression-ignition engine.
For example, JP2012-172665A discloses an engine which performs compression-ignition combustion of mixture gas within a cylinder when an operating state of the engine is within an operating range where an engine load is lower than a predetermined switching load, and which performs combustion by forcibly igniting the mixture gas within the cylinder with an ignition plug when the operating state of the engine is within an operating range where the engine load is higher than the switching load. With this engine, when performing the compression-ignition combustion, an exhaust valve is opened on exhaust stroke and also on intake stroke to introduce, into the cylinder, a part of exhaust gas discharged to the exhaust side, i.e., a so-called exhaust open-twice control is performed. The introduction of internal EGR gas by the exhaust open-twice control increases a compression-end temperature to improve ignitability in compression ignition and combustion stability.
Also, JP4159918B discloses an engine which performs, similarly to the engine in JP2012-172665A, the compression-ignition combustion within the low engine load range, and performs the spark-ignition combustion within the high engine load range. The engine is also configured to perform a fuel cut while decelerating, and when resuming from the fuel cut, even within the operating range to perform the compression-ignition combustion, the engine performs the spark-ignition combustion for a predetermined period of time, and then performs the compression-ignition combustion. In other words, since the temperature inside the cylinder becomes low during the fuel cut and the compression-ignition combustion cannot be performed stably when resuming from the fuel cut, the spark-ignition combustion is performed for the predetermined time period to secure the combustion stability and increase the temperature inside the cylinder.
As disclosed in JP4159918B, performing the spark-ignition combustion when resuming from the fuel cut is effective in view of the combustion stability; however, it causes degradation in exhaust emission performance and fuel consumption.